We have successfully etched a circuit board using our RepRap Mendel to draw the etch resist. We mounted a Sharpie ultra fine marker on our Mendel and printed up a RepRap motherboard. It worked way better than we anticipated.

The marker tip is ~0.7mm. We are reliably printing 0.7mm traces with 0.6mm clearance. Printing the resist took about 1 hour per coat. Feedrates could easily have been higher. Two coats is giving effective etch resist. We etched with MG ferric chloride acid* outside in 80 something F and it took about 20 minutes. After etching we washed the board with water to stop the process and removed the marker residue with acetone. Then off to the drill press for the tons of holes (I think drilling should have been done by Mendel too). This is in no way a substitute for spoolhead or other needed RepRap circuit technologies that are in the works by other RepRappers, but it works, and...  Yeah!

Pen mount derived from http://reprap.org/wiki/UniversalPenTouchProbe#Staedtler_Lumocolour_Pen. I used arhimed's Flexy Shaft Holder to clamp the marker, and hot glue to mount the copper clad board. Not getting the board level enough caused the marker tip to get smashed repeatedly and not mark the low spots. I threw together the springy pen mount, http://www.thingiverse.com/thing:3526. It allows the pen to travel vertically in its mount  > 1mm with bearable precision. It required a bit of post processing including hand drilling out  the upper pen bearing section to 10mm for the right fit, a hole 1.2mm for the ferrule holding the tip, and filing the threads for the lid. I ripped a spring from a retractable ball point pen, stretched it a little, and used handi-tack to secure it to the back of the marker. Ideally, the springy pen mount's lid's threads could be turned to a different angle to adjust the pressure on the spring. There was just enough adjustment when assembled. I tacked the threads in place w/ hot glue after it vibrated loose on the third or so print attempt.

Steps we used:

• Export traces from Eagle as an .png image @ 600dpi (Layers - Pads, Vias, Bottom, and Drill Aid).
• Open file with cad.py .
  in. per unit: 4.06
  file type: gcode
  maximum vector: 0.75
  tool diameter: 0.1666
  tool overlap: 0.309
  contours: -1
• Mendelize:
  mendelize.py -t 1.0 -z 60 -f 1800 ramps12.g > ramps12.gcode
• Mount single sided copper clad board to RepRap build surface w/ hot glue, use RepRap software to draw grid over board find height that marks the entire circuit board. Move build surface out Y to clean copper one more time. Set 0,0,0 to bottom left corner of circuit board.
• Use your favorite RepRap software to transfer the gcode to your printer, when it finishes print a second coat to make sure there is good coverage.
• Remove board and inspect. Use the edge of a small flat head screwdriver to scrape the ink off any traces that smudge together. Use marker to touch up any gaps in resist coverage.
• Etch in acid (find experienced help, or good instructions), drill holes.
• Solder components, and mount on RepRap :)

Requirements:
RepRap - http://reprap.org
Eagle or other Circuit board layout software - http://www.cadsoftusa.com/
cad.py - http://fab.cba.mit.edu/about/fab/
Metrix Create Space Mendelize - http://github.com/metrixcreate/mendelize
Printable PCB design - http://reprap.org/wiki/Arduino_Mega_Pololu_Shield
single sided copper clad board
Sharpie Ultra Fine Tip Marker
ferric chloride (use caution) pcb etchant and proper disposal method *see below
acetone (use caution) highly flamable, removes paints, and melts most plastics
plastic tubs
paper towels
water
drill press, unless you are set up to drill holes with your RepRap :)
solder iron and solder

Failures: Marker tip smashed, traces too thin, spacing too close, not enough tool lift drug ink between traces, "Sharpie Pen" ink did not dry?, etched through ink (printing resist twice fixed this), had wrong scale in cad.py (still may not be exact - but works), left cap off pen a million times

Resources: Of course RepRap. More specifically - Adrian's Pololu Electronics for pointing to the low hanging fruit, Vik's Olliver's suggestion of this method long ago, Matt Westervelt and Metrix Create Space for turning us on to cad.py and sharing their mendelize script, Tonok for his firmware we are using, LWK for reminding me more what it should look like, Nick McCoy's Gen3onAboard, MIT fablab for cad.py

* Chemicals: ALWAYS USE CARE WHEN HANDLING and DISPOSE OF CHEMICALS PROPERLY http://www.mgchemicals.com/techsupport/ferric_faq.html